Active arrays are complex electronic devices used extensively in military radars and communications. In addition, they are also found in some nonmilitary systems such as in equipment for weather forecast, space communications, and air-traffic control. In general, active arrays are very beneficial in systems where they replace critical mechanical operation with purely electrical operation. For example, the dish antenna of a conventional radar has a precise 3D geometrical design to generate a single narrow beam while an active array can generate multiple narrow beams with a planar construct. More importantly, one can change the beam direction of a dish antenna only by rotating the antenna in space appropriately but one changes the beam directions of an active array through electrical means alone without any mechanical movement. Therefore, the active array replaces mechanical beam forming and steering with electrical beam forming and steering. An important added benefit is a great increase in beam steering agility as electronic steering is substantially faster than mechanical steering. While physical laws of mechanical movement limit mechanical steering, electrical steering operates at the speed of electronic processing. The active arrays form and steer beams by phase shifting and magnitude scaling coherent signals applied to multiple radiating elements.
Despite the advantages mentioned above, the use of active arrays has been limited to high-end systems due a very high cost of manufacturing. Recently, in U.S. Pat. No. 8,611,959, all of which is incorporated herein by reference, a new method for designing low-cost active arrays has been described with potential wide-ranging commercial applications in cellular system, WiFi networks, and other wireless networks. Adding such low-cost active arrays to the wireless infrastructure will enable the capability to generate high-quality communication channels via electronically steerable beams. These beams may be directed in accordance with the mobile traffic density (e.g., see U.S. Ser. No. 14/799,935, filed Jul. 15, 2015, and entitled “Method of Adaptive Beam Placement in Wireless Systems,” incorporated herein by reference) or may scan the service area rapidly (e.g., see US Patent Publication 2012/0258754, entitled “Technique for Achieving High Average Spectrum Efficiency in a Wireless System, also incorporated herein by reference) to increase the network capacity significantly in both cases. This capacity boosting effect is enhanced if the beams are narrow, as generated by active arrays with a large number of elements. The narrower the beams the higher the quality of the signals exchanged between the base stations and the mobiles. Higher signal quality translates into higher network capacity. In addition, narrow beams allow frequency reuse by spatial division multiplexing, which also increases the system capacity.
A key requirement in the design of active arrays is electrical uniformity of all active elements in the array to maintain coherent and phase stable signals. Without very precise matching of the way elements respond to common electrical stimuli, it is not possible to generate or steer well-defined beams. In typical implementations in use today, high uniformity across the array is accomplished by using expensive architectures, expensive components, expensive assembly methods, and expensive calibration methods.
In order to illustrate the challenges of designing active arrays, consider a radio intended for independent operation, such as the radio inside a cell phone or of a base station. Usually, this radio is required to have excellent performance in terms of overall linearity and noise. However, the signal phase shifts due to time delays through the various radio components such as mixers, amplifiers, filters, etc. are of little relevance and can vary widely and randomly from unit to unit because they do not affect the performance of the system. In other words, if a large number of cell phone radios were tested for end-to-end signal phase shifting (signals delays), very few if any would have equal characteristics. Nevertheless, each cell phone radio works correctly as a single radio. Even the overall gain characteristic of the radio may vary moderately without a major loss in system performance.
The case of an active array is fundamentally different from the case above because all radios in the array must have the same overall phase and gain characteristics to a high degree of precision. This is a stringent design constraint, especially if the array must operate in wide environmental conditions, as is usually the case. Typically, in current art, the array electrical uniformity is realized by first building the radios with architectures and components, which have stable characteristics over wide environmental conditions. In addition, the array architecture includes means for adjusting the overall phase and magnitude characteristics of every array element to be able to compensate for unpredictable manufacturing and operational variations. Lastly, the array is calibrated during fabrication and often is re-calibrated at regular scheduled maintenance intervals. Re-calibration is necessary because in most cases the array electrical uniformity deteriorates slowly in time due to operation and environmental conditions.
Naturally, the design approach described above for conventional active arrays yields high manufacturing and maintenance costs. U.S. Pat. No. 8,611,959 discloses methods to design active arrays, which are significantly lower cost than conventional arrays due to their unique simplified architecture. However, the performance of even these arrays is susceptible to degradation due to manufacturing and operational variations such as temperature, humidity, and aging. Here we disclose methods to calibrate these arrays efficiently and automatically. Some of these methods can be applied without interrupting the normal operation of the array. Furthermore, these methods can be also applied to many other active arrays, including some conventional arrays such as digital arrays.